PF capacitor and Motor 1

[NickParker]

The questions are regarding switching sequence of PF capacitor with regards to both the LV and MV Motors.

A. Which is the better way to start a motor?
1. Starting a motor with the PF capacitor already switched ON, so that the capacitor assists in Motor starting.
(or)
2. Switching the PF capacitor with a slight time delay after the motor is switched ON, so that capacitor is protected from the Motor Inrush current

B. Similarly for switching OFF,
1. Should the capacitor contactor simultaneously switch OFF with the motor contactor?
(or)
The capacitor contactor switch OFF first and then the Motor contactor switches OFF to avoid over excitation problems.

C. Does simultaneous "Switching ON" and "Switching OFF" of contactor produces voltage transients?
D. How about the voltage transients when the capacitor contactor switching is delayed slightly?
E. How about the voltage transients when the capacitor contactor is switched OFF before the Motor contactor is switched OFF?

 

[LionelHutz]

Is this on a full-voltage start application or is there some other starting method?

On a full-voltage starter, it's common to connect the capacitors directly to the motor leads and switch both with the same starter.

With contactors for both, you should be able to simply put the coils in parallel and switch both at the same time. If the coil inrush is an issue then delay the capacitor by a small time amount when energizing but drop both contactors out at the same time.

 

[jraef]

If the capacitors are for power factor correction, they don't really aid in starting as much as you might think. So the typical method for Across-the-Line (DOL) starting is to switch the PFC capacitors along with the motor. As Lionel mentioned, if using some form of reduced voltage starting, that needs to be taken into account and done differently, depending on the type of starter. When turning OFF the motor, the caps can turn off at the same time, but make sure everyone involved understands their potential to store energy.

As to voltage transients, no transients can be created when disconnected from the line. So you WANT the capacitors to disconnect with the motor power. That does however factor in when looking at fault contribution by the motor, because when power is interrupted from up stream of that contactor, the caps will allow the motor to regenerate for longer.

There is a technique in starting large motors called "Capacitor Assisted Starting", but the caps are much much larger than PFC caps would be, so you must take them out of the circuit before the motor gets to full speed, otherwise you will severely over correct. So starting assistance caps need precice control of when they are applied and removed from the circuit. Different animal.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[GroovyGuy]

Hey Nick,
Historically I have always switched the capacitor with the motor (ie only one contactor for both the motor and the capacitor). This is the lowest installed cost for pf correction.
Also note;
1) The connection point for the caps should be on the line side of the O/L relay or CTs, otherwise the O/L device needs to compensate for the current produced by the caps.
2) Be wary if you have any large 6-pulse PWM ASDs on any MCC with pf caps. Obviously this is not a big deal with a (say) 24-pulse MV ASD.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

Fun with numbers;
If a motor draws 100 Amps at 80% PF.
It would take root(100A² - 80A²) = 60 Amps of capacitor current to correct to unity.
We often aim for 90% PF so roughly 30 Amps.
Starting current may be 600 Amps, quite reactive, minus 30 Amps of capacitor current = 570 Amps.
30A/600A - 5%. Not enough reduction in starting current to worry about.
A rigorous solution will yeild a slightly different figure that will still be too small to be of concern.
And the capacitors will not increase the starting current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NickParker]

I read that the Inrush current of capacitors are about 20 x rated current. Along with this there would be the inrush current of Motors too, Do the Instantaneous settings need to be compensated for the Inrush of capacitors too?

 

[LionelHutz]

I've seen it recommended to use an air core inductor to limit the capacitor inrush. Something like 6-8 turns of the wire around a 5-6" form.

 

[GroovyGuy]

Nick & Lionel,
You are correct in that the inrush current on the caps can be an issue, and in particular the ability of the over-current device to handle this transient current. However, this is not an issue with the O/L device as long as the caps are connected on the line side of this device (ie the O/L device does not see the inrush current).
I have pf caps installed into LV MCCs and the MCP tripping out every time the contactor was closed. Like Lionel's suggestion above, the solution was to provide a small amount of air-core inductance into the short length of wire between the starter bucket and the cap bucket. No issues after that.
BTW, it is common to install some inductive-reactance on MV cap banks as well.
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)